Patent GB-1388833-A (GB)
Title: Telecommunication line transmission power-feed protection arrangements
IPC: H04B3/44
Filed: 1971-05-20
Abstract: 1388833 Repeater power supplies PLESSEY TELECOMMUNICATIONS RESEARCH Ltd 12 July 1972 [20 May 1971] 16142/71 Heading H4R In a telecommunications line transmission system including at least one power feeding station, at least one power turn round repeater and a plurality of dependent repeaters, each power turn round repeater is provided with a pair of power feed pilot signal generators, one for each direction of traffic transmission, and each power feeding station is provided with power feed protection means arranged to cut off the power feed from that station over a corresponding transmission line leg when the pilot signal in the leg falls below a predetermined level and each power turn-round repeater is also provided with means actuated when power feed current and/or voltage variations exceed a predetermined threshold, said means when actuated inhibiting the power feed pilot signal generator for the leg. Fig. 1 shows a power feed station PFS, an exemplary dependent repeater DR and a power turn round repeater PTRR. Constant current generators CCG1, CCG2 supply D.C. power to inward and outward legs LT, LR via low pass filters, e.g. F1. At the turn round repeater the power legs are interconnected by low pass filter F4, repeater power circuit OPC1, a current detector CDC2, a centre tapped inductor having its centre top connected to earth via an unbalance voltage detector VUD1, a second repeater power circuit OPC2, a second current detector CDC2 and a low pass filter. For each leg there is also provided at the turn round repeater an oscillator such as PFO1 producing a pilot frequency signal of for example 30 KHz which is received at the power feed station by a power feed protection circuit PFA1. The operation of the circuit under several fault conditions is given. If a person connects himself in series with one of the power legs the pilot signal received at protection circuit PFA1 drops as does the current through current detector CDC1 assuming the resistance is sufficiently large that the constant current supplies cannot cope with the added resistance. The current drop in detector CDC1 causes the pilot signal generator to switch off, and the protection circuit PFA1 acts to cut off the generator CCG1 and to earth the leg (see Fig. 2, not shown). The protection circuit PFA1 is also arranged to operate due to the drop in pilot current caused by the increase in resistance of the leg alone. If a short or a resistive connection is made between the two legs LT, LR the current in both detectors CDC1 and CDC2 drops and both generators PFO1 and PFO2 are switched off thereby causing both generators CCG1 and CCG2 to be disabled and both legs to be earthed. This may be caused by a human being touching both leg conductors without earthing them. If either leg conductor becomes connected via a resistance to earth there will be an inbalance at the centre point of the inductor in turn round repeater PTRR. This is sensed by unbalance voltage detector VUD1 which causes the centre tap to be earthed. A current will then flow to earth at the fault point and the current detector CDC1 or CDC2 associated with the faulty leg will cause the coresponding pilot signal to be cut off and hence the respective constant current generator to be disabled. An automatic re-test facility is included whereby one second after the disabling of a supply, the supply is reconnected for 150 ms. If conditions have not returned to normal the supply is again disabled and a second retest is performed 1 second later. If again the retest is unsuccessful no further attempts are made automatically to restore power. Further attempts may be made manually. Relays are used in the control apparatus (Fig. 2, not shown) for the power supplies, but the relays could be replaced by solid state circuits.